1. Field of Invention
This invention relates to lead-free and cadmium-free, low temperature fired barium titanate-based dielectric compositions, and more particularly to barium titanate based dielectric compositions having relatively small proportions of guest ions dispersed within the barium titanate crystal matrix. Such a dielectric composition can be used to form multilayer ceramic chip capacitors having internal electrodes formed of palladium or silver, or mixtures or alloys of palladium and silver.
In particular, the present invention relates to a lead and cadmium free, low temperature fired dielectric ceramic composition system in which the dielectric constant does not alter from its base value by more than 15 percent over a wide temperature range. More specifically, this invention relates to a dielectric ceramic composition with a dielectric constant of over 4000 which is formed by firing a base ceramic oxide mixture at a temperature not exceeding 1150° C.
2. Description of Related Art
Multilayer ceramic chip capacitors (MLCC) have been widely utilized as miniature-sized, high capacitance and high reliability electronic components. In accordance with increasing demands for high-performance electronic equipment, multilayer ceramic chip capacitors also have encountered marketplace demand for smaller size, higher capacitance, lower cost, and higher reliability. Tightening environmental regulations militate in favor of capacitors, and hence dielectric compositions that are free of lead and cadmium.
Multilayer ceramic chip capacitors generally are fabricated by forming alternating layers of a dielectric layer-forming paste and an internal electrode forming paste. Such layers are typically formed by sheeting, printing, or similar techniques, followed by concurrent firing. Generally, the internal electrodes have been formed of conductors such as palladium, gold, silver or alloys of the foregoing.
Conventional ceramic dielectric formulations, such as disclosed in U.S. Pat. Nos. 4,816,430 and 4,882,305, lacking lead and cadmium often require sintering temperatures above 1250° C. The addition of lead or cadmium or their compounds, such as disclosed in U.S. Pat. Nos. 4,540,676 and 6,723,673 can reduce sintering temperatures from above 1250° C. to the range of about 1050° C. to about 1150° C. so that metal paste that contains 70 wt % or more silver and 30 wt % or less palladium may be used as inner electrodes.
The Electronic Industry Association (EIA) prescribes a standard for the temperature coefficient of capacitance (TCC) known as the X7R characteristic. The X7R characteristic requires that the change of capacitance be no greater than ±15% over the temperature range from −55° C. to +125° C. In actual practice, capacitance change of not more than ±20% from −30° C. to 125° C. is considered to be temperature stable. The percentage capacitance deviations in the X7R characteristics are measured from a reference temperature of 25° C.